1. Field of the Invention
This invention relates to a drive circuit for a cathode ray tube, and more particularly to a drive circuit for driving a cathode ray tube in response to a video signal.
2. Description of the Related Art
A flat type cathode ray tube wherein the fluorescent screen is formed in an inclined relationship at a comparatively small angle with respect to the center axis of the electron gun is conventionally known. FIG. 1 shows a drive system for such flat type cathode ray tube.
Referring to FIG. 1, the flat type cathode ray tube is generally denoted at 1 and includes a cathode electrode K, a first grid electrode G1, a second grid electrode G2 serving as an acceleration electrode, a third grid electrode G3 serving as a focusing electrode, and a fluorescent screen 2. A high voltage HV, which is obtained by rectifying a pulse voltage outputted from a flyback transformer, is applied to the fluorescent screen 2.
A directly heated electrode which operates with a low power and has a short rise time is adopted for the cathode electrode K as different from an indirectly heated electrode which is adopted in popular cathode ray tubes. A pulse voltage from the flyback transformer is supplied as a heater voltage 3 to the cathode electrode K in order to effect so-called pulse ignition.
In this instance, since a winding of the flyback transformer is connected to the cathode electrode K, the floating capacitance of the cathode electrode K increases, and where a cathode driving system wherein a video signal is applied to the cathode electrode K is adopted, the power loss of a video circuit increases. In particular, in order to enhance the frequency characteristic, the impedance of the output stage of the video circuit must necessarily be reduced so that the influence of the floating capacitance can be ignored, and to this end, an emitter follower configuration or a like configuration is adopted. However, this results in increase of the power loss of the video circuit.
Therefore, a first grid electrode driving system wherein a video signal is applied to the first grid electrode G1 which has a comparatively low floating capacitance is adopted. In particular, an NPN transistor 4 constituting a video amplifier is provided in the flat type cathode ray tube 1, and a video signal SV is supplied from a terminal 5 to the base of the transistor 4. The emitter of the transistor 4 is grounded by way of a parallel circuit of a resistor 6 and a capacitor 7, and the collector of the transistor 4 is connected to a power source terminal +B (for example, 50 V) by way of another resistor 8. A video signal obtained at a junction between the collector of the transistor 4 and the resistor 8 is applied to the first grid electrode G1 of the cathode ray tube 1 by way of a capacitor 9.
Meanwhile, a voltage source +B1 (for example, 900 V) is grounded by way of a series circuit of a variable resistor 10 for focusing, another variable resistor 11 for cutoff adjustment and a resistor 12. A voltage obtained at the movable terminal of the variable resistor 10 is applied to the third grid electrode G3 of the cathode ray tube 1 by way of a resistor 13. The movable terminal of the variable resistor 11 is grounded by way of a capacitor 14, and a voltage obtained at a junction between the movable terminal and the capacitor 14 is applied to the second grid electrode G2 of the cathode ray tube 1.
On the other hand, another voltage source +B2 (for example, 140 V) is grounded by way of a semi-fixed resistor 15 for sub-brightness adjustment, a resistor 16, a variable resistor 17 for brightness adjustment and another resistor 18 which constitutes a constant-current circuit. A voltage obtained at the movable terminal of the variable resistor 17 is applied to the cathode electrode K by way of a resistor 19.
Cutoff adjustment of the cathode ray tube 1 is performed by varying the voltages to be applied to the second grid electrode G2 and the cathode electrode K. In particular, the voltage to be applied to the second grid electrode G2 is varied by means of the variable resistor 11 to determine the cutoff voltage of the cathode ray tube 1, and the voltage to be applied to the cathode electrode K is varied by means of the semi-fixed resistor 15 to determine the sub-brightness of the cathode ray tube 1.
Since the voltage to be applied to the cathode ray tube 1 is varied by the sub-brightness adjustment performed by way of the semi-fixed resistor 15 or the brightness adjustment performed by way of the variable resistor 17 in this manner, the resistance value of the resistor 19 is set to a comparatively high value such as, for example, 100 K.OMEGA.. However, since a beam current which increases in proportion to the video signal flows into the cathode electrode K as well known in the art, the circuit impedance must be low. To this end, the cathode electrode K of the cathode ray tube 1 is grounded by way of the capacitor 20 to lower the ac impedance.
The flat type cathode ray tube 1 further includes a terminal 21 to which a horizontal blanking pulse signal HBLK is supplied and another terminal 22 to which a vertical blanking pulse signal VBLK is supplied. The terminal 21 is connected to the base of the NPN transistor 28 by way of a resistor 23 and a diode 24. The terminal 22 is connected to the base of the transistor 25 by way of another resistor 26. The base of the transistor 28 is grounded by way of a further resistor 27 and the emitter of the transistor 25 is grounded while the collector of the transistor 25 is connected to a junction between the transistor 4 and the resistor 8 by way of a still further resistor 28.
Since blanking pulse (positive pulse) signals HBLK and VBLK are supplied to the terminals 21 and 22 for horizontal and vertical blanking periods. respectively, the transistor 25 is turned on. Consequently, the voltage at the junction between the transistor 4 and the resistor 8 is reduced substantially to 0 V, and consequently, a blanking operation is performed.
In the flat type cathode ray tube described above, the voltage at the power source terminal +B is high, and consequently, the transistor 25 constituting the blanking circuit must have a high voltage-resisting property. Further, since a comparatively high current flows through the transistor 25 for an on-period of the transistor 25, the power loss increases. Further, since the blanking circuit is added as a floating capacitance for a video period which is an off-period of the transistor 25, the frequency characteristic is deteriorated. Furthermore, the flat type cathode ray tube 1 has a time constant provided by the capacitor 9 and the resistor 28 for coupling to the first grid electrode G1 and operates at a very high speed when the transistor 25 is on, but when the transistor 25 is off, since charging occurs at another time constant provided by the resistor 8 and the capacitor 9, the operation is low and the response characteristic to a video signal is deteriorated.
Further, in order to effect cutoff adjustment, the voltage to be applied to the second grid electrode G2 is varied by way of the variable resistor 11 to determine the cutoff voltage of the cathode ray tube 1, and the voltage to be applied to the cathode electrode is varied by means of the semi-fixed resistor 15 to determine the sub-brightness. Consequently, there is a problem in that the number of steps for the adjustment is great.
Further, in order to effect cutoff adjustment, the voltage to be applied to the cathode electrode K is varied, and to this end, the resistance of the resistor 19 is set to a high value. Therefore, the cathode electrode K is grounded by way of the capacitor 20 to lower the ac impedance. Consequently, there is another problem in that the number of components is great and the cost is high accordingly.
Further, in order to effect cutoff adjustment, the voltage to be applied to the second grid electrode G2 is varied, and accordingly, the resolution may possibly be deteriorated by such cutoff adjustment.